1. Field of Invention
The invention relates to a control system of a continuously variable transmission (hereinafter referred to as a “CVT”) for a vehicle, and more particularly, to a control system for regulating a torque transmitting capacity of the CVT on the basis of an estimated input torque.
2. Description of Related Art
As is widely known, a CVT for a vehicle is employed to continuously change the speed ratio and generally constructed so as to transmit power via a friction force generated by power transmitting members. It is, therefore, possible to improve the fuel efficiency and durability of the vehicle by controlling the friction force generated by the power transmitting members to a required value in accordance with an input torque to the CVT. When an internal combustion engine is employed as a driving motor, an estimated value of the input torque to the CVT can be calculated based on a throttle opening and engine speed both reflecting a torque required by a vehicle operator in accordance with a previously stored relationship. When an electric motor is employed as the driving motor, the estimated value of the input torque is calculated based on an electric current supplied for driving the motor (hereinafter referred to as “driving current”) in accordance with a previously stored relationship. In a belt type CVT serving to transmit power via a driving belt wrapped around a pair of pulleys having a variable effective diameter, the pressure applied to the driving belt by a hydraulic actuator, in accordance with the estimated input torque, is adjusted so as to control the transmitting torque capacity to an appropriate value. Also, in a traction drive CVT, which is a type of CVT including a pair of cones and a roller that is held therebetween and is rotatably supported around a rotation axis that lies in a plane perpendicular to those rotation axes, the holding pressure for holding the roller is controlled by a hydraulic actuator. As a result, the torque transmitting capacity of the CVT for the estimated input torque can be controlled to an appropriate value.
When the power of the vehicle is rapidly changed, such as when the output of the driving motor is rapidly increased upon kick-down operation of the vehicle operator who depresses an accelerator pedal deeply and rapidly, the hydraulic control performed by the CVT may fail to follow the rapid change. As a result, the CVT may fail to achieve a required hydraulic pressure or a required torque transmitting capacity, resulting in slippage between the power transmitting members. Such delay in the hydraulic control is caused by delays in mechanical operation for regulating hydraulic pressures. The delays in mechanical operation are attributable to the viscosity characteristics of operating oil in a hydraulic pressure control circuit or the structure of pressure control valves, and delays in data transfer among a plurality of electronic control units (hereinafter referred to as “ECUs”) respectively employed for engine control, hybrid system control, CVT control, and the like.
There has been proposed a control system disclosed in Japanese Laid-Opened Patent Publication No. 8-285021, in which even when the CVT is rapidly shifted upon rapid acceleration of the vehicle, the torque transmitting capacity of a belt type CVT is controlled to an appropriate capacity by applying an output line pressure corrected with a transient line pressure that is determined by an estimated engine torque and a shifting speed. In the aforementioned control system, however, the output line pressure is uniformly increased independent of a change rate of the estimated engine torque and therefore needs to be maintained at a level high enough to provide a sufficient magnitude of margin in order to prevent slippage of the belt. Accordingly, it occurs that the holding pressure becomes excessive in a normal operation state, resulting in an adverse effect to the fuel efficiency of the vehicle.